Constant-duty transponder

ABSTRACT

This invention relates to a high efficiency transponder of the type which receives and decodes interrogation pulse pairs and transmits reply pulse pairs. Filler pulses (squitter) are transmitted to achieve a constant duty operation. Squitter is initiated by a local noise pulse generator, not be receiver noise. Therefor, the countdown due to dead-time following decoding of the squitter is eliminated and recognition of genuine interrogations is not inhibited.

Unite Brisse et al.

States Patet s41 CONSTANT-DUTY TRANSPONDER 2,938,202 5/1960 Kirch et a]...343/6.8 R x Inventors: Jacques Danie pp Brim, 3,178,706 4/1965 Clock..343/6.8 LC

gg g J z Primary Examiner-Malcolm F. l-lubler F ogne' l ancoun 0t 0Attorney-C. Cornell Remsen, Jr., Walter J. Baum, rance Paul W.Hemminger, Charles L. Johnson, Jr., Philip [73] Assignee: InternationalStandard Electric Cor- BOltOn, Isidore 8 Edward Goldberg and Pfl al on,New York, NY. Menotti J. Lombardi, Jr.

[22] Filed: July 23, 1970 [57] ABSTRACT [211 App! NOJ 57668 Thisinvention relates to a high efficiency transponder of the type whichreceives and decodes interrogation [52] us. Cl ..343/6.8 LC Pulse Pairsand transmits reply Pulse P Filler P [51 Int. Cl .0015 9/56 (squmer) aretransmitted achieve a ccmmm [58] Field of Search 343/6 8 R 6 8 LC 106 Rduty operation. Squitter is initiated by a local noise pulse generator,not be receiver noise. Therefor, the [56] References Cited countdown dueto dead-time following decoding of the squitter is eliminated andrecognition of genuine UNITED STATES PATENTS interrogations is notinhibited.

3,454,948 7/1969 Reinagel ..343/l06 R 8 Claims, 2 Drawing Figures J70 355 6 M /4 /5 M a AEcnrsn 8 1 8:14) LINE l/b flA/rE/V/Vfl Mezxmges fq 2fiflfl'fiioe PYA'TE N IED DEC 19 e972 SHEET 2 OF 2 G 7 (5% AtlorneyCONSTANT-DUTY TRANSPONDER BACKGROUND OF THE INVENTION The presentinvention relates to transponder circuits used in radio navigationsystems and more particularly to circuits which improve the responseefficiency of a high transmission rate transponder.

In known radio navigation systems, such as the DME (Distance measuringequipment) an aircraft transmitter sends, to all bearings, interrogation(inquiry) pulses on a carrier frequency which is characteristic of aresponder radio-beacon located on the ground, also known as atransponder.

The transponder receives the inquiry signal, detects it and uses it toremodulate a transmission of pulses, termed response pulses, the carrierfrequency of which differs from the received frequency. The responsesignal transmitted in this way by the transponder is received by theinterrogating aircraft.

The aircraft radio equipment measures time t which elapses between thetransmission of the inquiry signal and the reception of the responsesignal. This period of time increases linearly according to the distanceD between the aircraft and the antenna of the transponder. Thetransponder, imposes given delays for the transmission and theprocessing of the transmitted or received signals. That is, thetransponder equipment is adjusted via adjustable delay lines or shiftregisters, for instance, so as to introduce a systematic delay period tdistance D is then: D V2 c (t t where c is the speed of light.

In another well-known system, namely the TACAN (Tactical AirNavigation), the interrogating aircraft receives, from the transponder,two pieces of information allowing it to determine its distance D andits bearing with respect to said transponder.

The distance measuring function in the TACAN is effected by means verysimilar to those implemented in the DME system in its original form;thus in the following description we shall confine ourselves, to thecharacteristics of the present TACAN as defined for instance in the MILstandards published by the United States Defense Department.

In TACAN systems, inquiry interrogation signals like response signalsare constituted by pairs of pulses; the two pulses which make up a paireach have an average duration of (t,,)=3.5 microseconds and theirleading edges are separated by a time t,, which, in most currentapplications, amounts to 12 microseconds.

As was pointed out above, a systemic delay t of 50 microseconds isintroduced into the transponder equipment between the second (or first)pulse of a pair of received inquiry pulses and the second (or first)pulse of a pair of transmitted response pulses.

One of the basic characteristics of the TACAN system lies in the factthat the transponder transmitter, regardless of whether it is receivinginquiries from aircraft or not, continuously transmits (each second) Npairs of pulses (N 2 700 in known systems). In the absence of inquiriesthese N pairs of pulses filling pulses are generated by a localgenerator of N noise pulses distributed randomly in time and whichsimulate inquiry recognition pulses and response triggering pulses whichnormally appear at the output of the decoder. If genuine inquires are infact detected and recognized, they replace some of the randomlydistributed noise pulses. A pulse counter located at the output of thecoder acts upon, via an automatic control device, the noise pulsegenerator, so as to reduce the number of pulses it generates per second.

In these conditions, the transponder transmitter transmits (each second)a constant number N of pulse pairs without regard to whether theyrepresent recognition of a genuine inquiry or merely pulses from thelocal generator. The main advantage of this system is that it makes itpossible to operate the transponder radio-frequency transmitter at aconstant average power, thus facilitating operation and maintenance. Afurther advantage is that, in the case of the TACAN, the filling pairsof pulses generated by the transponder local pulse generator providealkaircraft, even where part of their distance measuring equipment islacking, with bearing information with respect to the transponder.

It should be noted, incidentally, that when a large number of aircraftare submitting inquiries, the number of real inquiries can reach orexceed the value of N per second. The filling pulses then disappear andthe automatic control associated with the counter of response pulsepairs acts upon the receiver of the transponder so as to make it refuselower level inquiries. The modes of implementation of these deviceswhich limit the number of responses to inquiries to N per second arewell known and are not within the scope of the present invention.

The TACAN system has another limitation constituted by minimum dead timet,,, which must separate two successive pairs of pulses transmitted bythe transponder.

This dead time is necessary owing to various technical considerationsarising for instance out of the operation of the transpondertransmitter, or that of the aircraft receivers which use the fillingpulse pairs to measure the bearing. This dead time is also necessary ina case where a real inquiry may be followed by ghost inquiries, which infact correspond to multiple radioelectric paths between the aircrafttransmitter and the transponder receiver.

In current known systems, inhibition gates operating during time t,,which follow the appearance of a response triggering pulse prevent, onthe one hand the decoding of subsequent genuine inquiry pairs or eventhe operation of the receiver and, on the other hand, bar access to theinput of the coder for noise pulses generated by the local generator. Inthe current TACAN, dead time r equals 60 microseconds.

In some current systems, when inadequate decoupling of the transponderreceiver and the transponder transmitter occurs, it is necessary, inorder to ensure operational reliability of the receiver, to inhibit thelatter for the duration of the response pairs transmission. Thisinhibition must begin approximately at time t t, which follows thetriggering off of the coding process and continues up to time t Incurrent systems, time t (50 microseconds) is shorter than dead time t(60 microseconds) defined above, so that inhibiting the receiver fortime t, also ensures its protection during transmission.

Said dead time t reduces the efficiency of response (r) of thetransponder, defined as the ratio of the number of responses tointerrogations, to the total l060ll 0817 number of N inquiries which thetransponder can receive mathematically, r may be:

In present implementations, r also measures the probability of anyinquiry giving rise to a response from i the transponder. In otherwords, if the transponder receives inquiries from a small number ofaircraft sending n inquiries per second (n being much lower than N) anddoes not have the means to process inquiry recognition pulsesdifferently from the pulses generated by the local generator, efficiencyr is that obtained when N inquiries occur.

With the usual values indicated above (N 2 700 and t,,, 60microseconds), r has a maximum value of 85 percent.

Now, to increase the quantity of distance information per second sentout to a group of aircraft interrogating a transponder, or to increasethe accuracy of bearing measurements in the area covered by a TACANtransponder, it is necessary to increase the transmission rate of theresponse pulse pairs. If N is increased to 10,000 and if a dead timet,,, of 60 microseconds is retained, the response efficiency r'of thetransponder is less than as 60 percent of its maximum, which is too lowto be compatible with smooth operation of the aircraft equipment.

This can be remedied simply by decreasing t,,, to an extent compatiblewith the proper utilization of the system, for instance if t 35microseconds, r has a maximum value of 74%.

However, even that improvement is still insufficient. In many modernapplications nearly 100 percent efficiency is required where the numberof interrogating aircraft is small.

The present invention therefore offers devices which, associated withreception, decoding, coding and transmission circuits of the typenormally used in transponders, increase response efficiency and wherebyan almost 100 percent efficiency can be achieved where the number ofinterrogations is low. The devices offered by the present invention makeit possible to give top priority to interrogations and to thecorresponding response pulse pairs over noise pulses from the localgenerator and the filling pulse pairs they generate. They allow for:

minimum spacing (or dead time) between the transmitted pulse pairs;

where required, inhibition protection of the reception circuits duringthe transmission duration of each pair only;

response efficiency compatible with the smooth operation of the aircraftequipment.

The transponder equipment, (which is well-known in itself), into whichwe have incorporated the devices according to the invention,schematically comprises wellknown components which are as follows:

a radio frequency receiver and detected video frequency pulse shapingcomponents;

a decoder for recognizing pairs of pulses at t, intervals according tothe agreed inquiry mode;

a logic device which inhibits the decoder input for dead time t,,, afterthe recognition of an inquiry pair;

a delaying line (t T t whereby a systemic internal delay t is introducedbetween the second (or first) pulse of the received interrogation andthe second (or first) pulse of the response transmitted (T equals thesum of unavoidable, but, well known, delays arising from certaincomponents of the transponder, in particular from the receptionintermediate frequency stages and from the transmitting stages);

a coder for response pulse pairs separated by a t, in-

terval;

devices to shape said response pulses, a radio frequency energymodulator and a radio frequency transmitter;

a generator yielding a'maximum of N random dis- I tributed noise pulsesper second at minimum inter vals equal to t,,,, each noise pulsegenerating a pair oftilling pulses;

an automatic control system controlled by a transmitted pulse paircounter whereby the number of pulses generated per second by the noisegenerator can be reduced to N n if, during the same time, n pairs ofpulses corresponding to genuine responses are transmitted.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a means for preventing noise pulses produced by the localgenerator from being introduced into the response pulse coding circuits,during time T which follows the recognition of an inquiry pair. T is atleast equal to the time which elapses between the recognition of aninquiry and the transmission of the first corresponding responsepulse,therefore T is at least equal to delay (t T t of the delay linewhich precedes the coder.

According to another characteristic of the invention, noise pulsesgenerated by the local generator are directly introduced at the input ofthe coding circuits and not at the input of the delay line whichprecedes said coding,circuits.

Due to the aforementioned characteristics of the invention, thetransmission of filling pulse pairs is prevented at least until agenuine response has been given to the inquiry.

According to a further characteristic of the invention, means areprovided whereby noise pulses generated by the local generator arebarred entry to the coding circuits during dead time t,,, which followsthe generation of the first genuine pulse or-ofa filling pair.

In some transponder implementations, the decoupling between thetransmitter and the receiver is inadequate in which case the lattershould be inhibited for a given period.

According to a modification of the invention, means are provided wherebythe receiver is only inhibited for the duration of the transmission of aresponse pulse pair, i.e. for duration t equal to t, 22

In order to appreciate the advantages of the invention, one shouldcompare the response efficiencies obtained in the various cases.

In current prior art type systems, response efficiency for a smallnumber n of inquiries is independent of said number n and is given inthe following relation:

In systems comprising devices according to the invention including thosewhich make it possible to inhibit the receiver for the transmissionduration, efficiency stands at:

Finally, in systems comprising devices according to the invention butlacking receiver inhibition capabilities, efficiency is:

r,, may be almost 100 percent where the number of inquiries is low.

According to a preferred embodiment of the invention,.the second pulseof each decoded inquiry is sent to the input of a first delay shiftregister, the total delay of which is approximately t T, the last stagesof which serve as a coder, the first response pulse coming out of theoutput of the stage which corresponds to a delay (t T t similarly eachnoise pulse yielded by the local generator is sent to the input of asecond shift register yielding a delay t,-, and the sole purpose ofwhich is to code filling pulses. The first pulse outputs of the tworegisters are united via a first OR gate and the second pulse outputs ofthe said registers are united via a second OR gate.

This embodiment is advantageous since internal delay t of thetransponder can be achieved with the accuracy generally required.

The objects and characteristics of the present invention will becomeclear from the following description and reference to the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an operational diagram ofa TACAN system transponder incorporating the devices according to theinvention.

FIG. 2 shows a more detailed diagram of an embodiment of the devicesaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Known componentsnot necessaryto the understanding of the principle of the invention have not beenincluded in the two figures. In particular the antenna array whichproduces a revolving response pattern characteristic of the TACAN andthe identification signal generating circuits have been omitted.

The equipment shown in FIG. 1 has a transmitting and receivingantenna 1. The radio frequency signals it receives are directed by alink 2, and its branch 2a, to a receiver 3. Video frequency pulses andin particular, interrogation pulse pairs arrive via wire 4 at shapingcircuit 5. The output of 5 is connected via wire 6 to an input of ANDgate 7 which also has an inhibition input 8. The output of 7 isconnected via lead 9 to an interrogation pulse pair decoder 10. A lead11 and its branch 11a connect the output of to the input of monostablemultivibrator 12, the output of which is connected via lead 13 toinhibition input 8 of AND gate 7.

Lead 11 connects the output of 10 to the input of delay line 14, theoutput of which is connected via wire 15 to one of the two inputs ofORgate 16. A lead 17 links the output of 16 to the input of response pulsepair coder 18 made up, for instance, of a shift register.

A random distribution noise pulse generator 19, sends, via lead 20,noise pulses to an input of AND gate 21 which also has two inhibitioninputs 22 and 23.

The output of 21 is connected via lead 24 to the second input ofOR" gate16. A branch 17a of lead 17 is connected to pulse counter 25 which, viaautomatic control circuits represented by unit 26, makes it possible tomaintain the number of noise pulses generated per second by generator 19at the required value.

Via branch 17c, the output of gate 16 is connected to a monostablemultivibrator 27, the output of which is connected to inhibition input22 of gate 21. Via a branch 11b the output of decoder 10 is connected tomonostable multivibrator 28, the output of which is connected toinhibition input 23 of gate 21.

A branch 17b channels the first pulse of a response or filling pair toone of the inputs of OR" gate via lead 29 the second pulse of theresponse or filling pulse pair leaving coder 18 arrives at the otherinput of said gate 30. The output of 30 is connected via 31 to unit 32which represents the shaping, modulation and transmission circuits ofresponse or filling pulse pairs. The radio frequency signal carryingsaid pairs reaches antenna 1, via branch 2b of link 2.

Via a branch 17d of wire 17, gate 16 is connected to the input of amonostable multivibrator 33, the output of which is connected via wire34 to inhibition input 35 of receiver 3.

All the components mentioned above are well known in the present stateof the art and there are numerous possible embodiments for each one ofthem. The technician will recognize in the general layout, most of thesecomponents of the distance measuring circuits of a TACAN Transponder.

New components and new layouts which allow for the implementation of theinvention are shown in the center of FIG. 1. Said new componentscomprise, in particular, monostable multivibrator 28 and AND" gate 21;the most striking new feature in the layout being the fact that theoutput of noise pulse generator 19 is connected via wire 20, gate 21,wire 24-, gate 16 and wire 17 to the input of coder 18 so that saidnoise pulses are not delayed in delay line 14.

Another novel feature, which is required only if the decoupling betweentransmitter 32 and receiver 3 is not adequate, is monostablemulti-vibrator 33.

There follows a description of the operation of the equipment shown inFIG. 1 with special importance attached to the components according tothe invention.

A radio frequency signal carrying a pair of interrogation pulsesseparated by time t,, equal for instance to 12 microseconds, is pickedup by antenna 1 and via connections 2 and 2a, reaches receiver 3 at atime when the latter is not inhibited by its input 35.

The pair pulses, thus detected in 3, are shaped in 5 then, via 6, reachthe input of AND" gate 7. If the second input 8 of gate 7 is notinhibited, the two pulses reach decoder 10 via wire 9.

When 10 recognizes an inquiry pair, a pulse hereafter termed masterpulse appears on lead 1 1.

Since the circuits located ahead on decoder 10, and especially theintermediate frequency stages of receiver 3, generate delay T, themaster pulse appears at the end of T, following the arrival of thesecond inquiry pulse in receiver 3.

The master pulse is delayed for time (t T t in delay line 14. Theinternal delay of the transponder according to the specifications of theTACAN, has a precise value of 50 microseconds. Delay T, caused by thetransmission circuits incorporated in unit 32. In practise, T isapproximately 2 or 3 microseconds and t, is 12 microseconds. Under theseconditions, the delay of line 14 is 35 or 36 microseconds. The masterpulse delayed in 14 reaches, via lead 15, OR gate 16 and thence by lead17, the input of coder 18, and through the same channel plus branch 17b,one of the inputs of OR" gate 30. The pulse exiting from coder 18 via 29reaches the other input of 30. The two pulses separated by interval t,which stand at the inputs of 30 correspond to the response pulses to theinquiry. After processing in unit 32, they are transmitted by a radiofrequency carrier wave emitted by antenna 1.

Immediately after decoder recognizes an inquiry pair, and input 8 ofgate 7 is inhibited, via branch lla, monostable multivibrator 12 andwire 13. The inhibition duration of 8, determined by the characteristicsof 12, is at least equal to dead time t,,,, i.e. to the minimumacceptable interval between two successively transmitted pairs ofpulses; in the example described, r is 35 microseconds. I

Under these conditions, a second inquiry received in 3 during thisinhibition period would not be able to reach decoder 10.

As soon as 10 recognizes the first inquiry pair, input 23 of AND gate 21is inhibited, via branch 11b and monostable multivibrator 28. Theinhibition duration of 23, determined by the characteristics of 28, isequal to (t T t i.e. to the delay ofline 14.

The master pulse delayed in 14 triggers via wire 15, gate 16 and branch17c, monostable multivibrator 27, which inhibits input 22 of gate 21.The inhibition duration as determined by the characteristics of27, isequal to t,,,, i.e. in the example chosen, 35 microseconds.

We therefore find that after a pair of inquiry pulses is recognized atthe output of 10, the pulses yielded by generator 19 cannot reach coder18 for a period equal to t t,,, T t, i.e., in the example chosen,approximately 70 microseconds. The generation of a response to arecognized inquiry is therefore unhindered by the presence of noisepulse generator 19. If it is not necessary to inhibit receiver 3 for theduration of response transmission, the response efficiency of thetransponder being almost I00 percent when the number of inquiries persecond is relatively low.

It can also be seen that apart from the periods during which thetransponder is busy coding and transmitting genuine responses, thetransponder operates according to the well known process i.e. ittransmits filling pulse pairs.

In some cases, however, it is necessary to protect receiver 3 for theduration of the transmission. To achieve this, the pulse standing at theoutput of 16 triggers monostable multivibrator 33. Via wire 34, input 35of receiver 3 is inhibited. The inhibition duration as determined by thecharacteristics of 33, is equal to t, 2%,, r measuring the width of eachresponse pulse; in the example chosen t, 2r has a value of 19microseconds.

In this case response efficiency for a transmission rate of 10,000 istherefore 84 percent whereas in known systems, with a microsecond deadtime 2 efficiency does not exceed 74 percent.

A specific embodiment example of the devices according to the inventionand in particular delay line 14 of coder 18, is shown in FIG. 2.

Components of FIG. 1 not necessary to the understanding of the followingdescription have not been included in FIG. 2. Those reproduced withoutany structural alteration bear the same references as in FIG. 1. Y

New components which characterize this particular example embodiment ofthe devices according to the invention, are in a rectangular boxoutlined in dots and dashes (36).

The master pulse which emerges from decoder 10 after the recognition ofan inquiry arrives, via wire 11, at the input of monostablemultivibrator 37 which can be finely adjusted, between 0 and lmicrosecond'for instance, and makes it possible to set the internaldelay of the transponder at the required value t (50 microseconds). Theoutput of 37 is connected via wire 38 to the write input e of aflip-flop 39. The output s, of 39 is connected to one of the inputs ofan AND" gate 40. The other input of 40 is connected via wire 42 to aclock 41 having a frequency of 20 MHz for instance. The output of 40 isconnected to a first frequency divider 43 which lowers the frequency ofclock 41 to a suitable value, thus creating a time base with an interval(or step) of one microsecond, for instance. The output of divider 43 isconnected via wire 44 to the timing inputs H of a first shift register45. The pulse input E of 45 is connected by a branch'38a of lead 38 tothe output of 37. The register has p stages; each stage having itsoutput but the only outputs used being output 8,, and final output 8,.The delay occurring between the master pulse entering at E in register45 and the delayed pulse which emerges at 8,, must be equal to t T i.e.slightly less than microseconds. Consequently as the steps last Imicrosecond, the number p of stages is slightly lower than 50. Thedetermination ofp will be discussed later.

Output S of register 45 corresponds to a delay (z T t i.e. in this case35 to 36 microseconds.

A branch 42a of lead 42 connects the output of clock 41 to a secondfrequency divider 46 which lowers the frequency of 41 to a suitablevalue thus creating a time base with an interval (or step) also of lmicrosecond. The output of divider 46 is connected via lead 47 to thespeed outputs H of a second shift register 48. The pulse input E of 48is connected via wire 24 to the output of AND gate 21, the function ofwhich was described with reference to the components of FIG. 1.

Register 48 has q stages; each stage has its output but only output S,of the first stage and output 8, of the last stage are used. The delayoccurring between the pulses emerging respectively from S, and 8' mustbe equal to t, i.e. l2 microseconds; consequently q equals 13.

Wire 43 links output S of register 45 to erasure input e; of flip-flop39. Outputs 8,, of 45 and S',, of 48 are linked respectively by a branch49a of lead 49 and a lead 50 to the two inputs of an OR gate 51.

l060ll 0820 Outputs 8,, of 45 and S,, of 48 are linked respectively vialead 15 and a wire 52 to the two inputs of OR gate 16 already mentioned.

Finally, as explained when describing the devices of FIG. 1, the outputsof 51 and 16 are linked in OR" gate 30; at the output of 30 stand theresponse pulse pairs or filling pairs.

There follows a description of the operation of the circuits shown inFIG. 2.

it is assumed that before a pulse appeared at the output of 10,flip-flop 39 is in state (s 0). Gate 40 is closed and no timing pulsesare applied to first register 45.

On the other hand, the timing pulses produced by divider 46 are appliedto second register 46. If gate 21 is not inhibited the noise pulsesgenerated by 19 reach input E of 48. They move forward in 48 at the timebase pace. At the outputs S", and 8, stand at an interval of 12microseconds, these two pulses comprising a filling pair.

As soon as an inquiry interrogation is recognized as such, the masterpulse emerging from locks, as has already been noted, gate 21 for aperiod (1,, T t,) (35 to 36 microseconds). The master pulse is delayedby monostable multivibrator 37 for a period between 0 and l microsecond;via wire 38, it turns 39 into state 1 (s 1). Gate 40 opens. Via wire 42and gate 40, clock pulses 41 reach divider 43. Timing pulses, via wire44, are applied to inputs H of register 45.

Via branch 38a, the master pulse is applied to input E of register 45.

The interval between the arrival of the first timing pulse and that ofthe master pulse at register 45 is at least equal to a step of clock 41,i.e. in the example chosen: 0.05 microsecond.

The pulse moves forward in register 45, according to the time base pace,reaches output 8,, after a period (t T t i.e. 35 to 36 microseconds) andarrives at output S after a period (t T i.e. 47 to 48 microseconds).

The two pulses (occurring at an interval of 12 microseconds) whichemerge at S and S, are linked via gates 16 and 51 in gate 30. Theyconstitute the response pair to the inquiry.

The pulse which emerges at S,,, via wire 49, triggers 39 which revertsto state 0 (s 0) and gate 40 closes, awaiting a new master pulse.

A further word about the multivibrator 37 is in order here. Given thequantic" nature of the operation of a shift register, it is impossibleto achieve a delay which is not a multiple of the step, i.e., in thepresent case, an integral number of microseconds. lt is thereforenecessary to incorporate a balancer which adds delay T to an integralnumber of microseconds. This is precisely the role of multivibrator 37.

Although the principles of the present invention have been describedhereinabove with reference to a particular example or embodiment, itwill be clearly understood that the said description has been only madeby way of example and does not limit the scope of the invention.

We claim:

1. A transponder for transmitting a coded, spaced response pulse pairwhenever coded interrogation pulse pair having predetermined spacing isreceived and recognized, comprising:

means including an antenna and receiver for receiving said interrogationpulse pairs;

a decoder responsive to the output of said receiver for recognizing saidcoded interrogation pulse pair and for generating an output signalrepresentative of said recognition; response pulse pair encoding andtransmitting means;

delay means responsive to and in series with said decoder output forintroducing a predetermined system delay between said recognition andtransmission of said response pulse pair;

a random distribution noise pulse generator and control means forgenerating a predetermined maximum number of randomly distributed noisepulses per unit of time, said random pulses being applied to saidencoding and transmitting means for transmitting pulse pairs from saidtransponder substantially independently of said decoder output signal;

and first inhibiting means responsive to said decoder output signal forpreventing said randomly distributed pulses from reaching said encodingand transmitting means during a predetermined dead time after eachtransmitted response.

2. Apparatus according to claim 1 including second inhibiting meansassociated with said decoder for inhibiting the generation of saiddecoder output signals for a period at least equal to said predetermineddead time after each of said decoder output signals corresponding to aninterrogation recognition, and said first inhibiting means are definedas being responsive to both said delay means output and said randompulses to inhibit said encoding and transmitting means after eachtransmission, whether said transmission was initiated by said recognizedinterrogations or by any of said random noise pulses.

3. Apparatus according to claim 2 in which said control means associatedwith said noise pulse generator is responsive to the signals at theinput of said encoding means, whereby the number of noise pulsesgenerated is controlled so that the sum of said noise pulses and decoderoutput pulses does not exceed said maximum predetermined number.

4. Apparatus according to claim 3 including means responsive to saidencoder input for inhibiting the operation of said receiver for a secondpredetermined time following each interrogation recognition outputsignal from said decoder.

5. Apparatus according to claim 4 in which said second predeterminedtime is at least equal to the time separation between the pulses of aresponse pair plus the durations of the two pulses of said responsepair.

6. Apparatus according to claim 4 in which said second predeterminedtime is substantially equal to twice the transmitted pulse width plusthe spacing between pulses of said response pulse pair.

7. Apparatus according to claim 1 in which said delay means and saidresponse pulse pair coder comprises a first shift register having atotal overall delay of said predetermined system delay plus the timebetween pulses of said response pulse pair.

8. Apparatus according to claim 7 in which said random distributionnoise pulse generator includes a second shift register having a totaldelay equal to said 'zg g BNITEI) STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,705,992 Dated December 19, 19?;

Inventor) ,Ia cques Daniel Phillippe Brisse and Jean-Claude Ioguet It iscertified that error appears in the above-identified patentand that saidLetters Patent are hereby corrected as shown below:

On the Title Page add the following:

[30] Foreign Application Priority Data August 8, 1969 France 6927328Signed and sealed this 11th day of June 197 v (SEAL) Attest:

EDWARD M.FLETGHER,JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents 3 1';NITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,70 ,992 Dated December 19, 19?;

Inventor) Jacques Daniel Phillippe Brisse and lean-Claude Ioguet It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

On the Title Page add the following:

[30] Foreign Application Priority Data August 8, 1969 France 6927328Signed and sealed this 11th day of June 1971p,

(SEAL) Attest:

EDWARD'MJLETCHERJR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. A transponder for transmitting a coded, spaced response pulse pairwhenever coded interrogation pulse pair having predetermined spacing isreceived and recognized, comprising: means including an antenna andreceiver for receiving said interrogation pulse pairs; a decoderresponsive to the output of said receiver for recognizing said codedinterrogation pulse pair and for generating an output signalrepresentative of said recognition; response pulse pair encoding andtransmitting means; delay means responsive to and in series with saiddecoder output for introducing a predetermined system delay between saidrecognition and transmission of said response pulse pair; a randomdistribution noise pulse generator and control means for generating apredetermined maximum number of randomly distributed noise pulses perunit of time, said random pulses being applied to said encoding andtransmitting means for transmitting pulse pairs from said transpondersubstantially independently of said decoder output signal; and firstinhibiting means responsive to said decoder output signal for preventingsaid randomly distributed pulses from reaching said encoding andtransmitting means during a predetermined dead time after eachtransmitted response.
 2. Apparatus according to claim 1 including secondinhibiting means associated with said decoder for inhibiting thegeneration of said decoder output signals for a period at least equal tosaid predetermined dead time after each of said decoder output signalscorresponding to an interrogation recognition, and said first inhibitingmeans are defined as being responsive to both said delay means outputand said random pulses to inhibit said encoding and transmitting meansafter each transmission, whether said transmission was initiated by saidrecognized interrogations or by any of said random noise pulses. 3.Apparatus according to claim 2 in which said control means associatedwith said noise pulse generator is responsive to the signals at theinput of said encoding means, whereby the number of noise pulsesgenerated is controlled so that the sum of said noise pulses and decoderoutput pulses does not exceed said maximum predetermined number. 4.Apparatus according to claim 3 including means responsive to saidencoder input for inhibiting the operation of said receiver for a secondpredetermined time following each interrogation recognition outputsignal from said decoder.
 5. Apparatus according to claim 4 in whichsaid second predetermined time is at least equal to the time separationbetween the pulses of a response pair plus the durations of the twopulses of said response pair.
 6. Apparatus according to claim 4 in whichsaid second predetermined time is substantially equal to twice thetransmitted pulse width plus the spacing between pulses of said responsepulse pair.
 7. Apparatus according to claim 1 in which said delay meansand said response pulse pair coder comprises a first shift registerhaving a total overall delay of said predetermined system delay plus thetime between pulses of said response pulse pair.
 8. Apparatus accordingto claim 7 in which said random distribution noise pulse generatorincludes a second shift register having a total delay equal to said timebetween pulses of said response pulse pair, said shift register beingconnected to generate the second pulse of said response pair from thefirst pulse of said pair at the input of said second shift register.